The present invention relates generally to electron beam tubes having dual beams and associated deflection means, and more particularly to single scan expansion mesh cathode ray tubes having dual electron beam guns that provide asymmetrical horizontal scanning.
In cathode-ray tubes having dual electron beams for independent, multitrace operation, it has been a common practice to provide a pair of electron guns mounted in a stacked configuration such that one gun is disposed above the central axis of the tube, and the other gun disposed below the central axis, both guns being mounted in the same vertical plane so that the horizontal angles of the two electron beams swept horizontally across the tube screen are equal. Since the screen is transverse to the central axis of the tube and the guns are mounted off-axis, the guns are angled toward the central axis so that each gun can scan the entire screen. However, this angling distorts the scanned display pattern from a rectangle to a trapezoid, which can easily be corrected by adjusting the horizontal deflection plates to be nonparallel from side to side thereby affecting the horizontal sensitivity as the tube is scanned vertically. If a divergent post deflection acceleration (PDA) field is used and full overlap of scan is desired, the guns have to be angled quite steeply, resulting in a large diameter cathode-ray tube which takes up useful space.
U.S. Pat. No. 3,819,984, assigned to assignee of the present application, teaches the concept of mounting a pair of electron guns parallel to each other in a horizontal plane in order to reduce the tube neck diameter. However, the use of a single scan expansion mesh in a cathode-ray tube having parallel electron guns and associated deflection structures produces the undesirable effect of causing the two electrical centers of the guns to be widely separated, for example, as much as eight centimeters, and introduces a vertical trace bowing problem at the center of the display screen.